The present invention relates to a device for carrying out a heterogeneously catalysed reaction in which a suitable reaction mixture is fed onto a catalyst, and to a process for producing a catalyst which is suitable in particular for use in a device of this nature.
An example of a heterogeneously catalysed reaction is the generation of hydrogen from hydrocarbon or alcohol, in particular methanol (methanol reforming), in which a reaction mixture comprising hydrocarbon or alcohol and water is fed onto a catalyst. Further examples are the reduction of carbon monoxide levels so that carbon dioxide is liberated in a so-called hydrogen shift reaction, the oxidation of carbon monoxide in which a CO-containing gas and an O2-containing gas are fed onto a catalyst and a combustible starting material is burnt with the addition of an O2-containing gas in a catalytic burner.
Obtaining hydrogen from methanol is based on the overall reaction CH3OH+H2Oxe2x86x92CO2+3H2. In practice, to carry out this reaction a reaction mixture comprising the hydrocarbon and steam is guided past a suitable catalyst during heating, in order to produce the desired hydrogen in a two-stage or multistage reaction sequence. A two-stage methanol reforming device of this nature is known from EP 0,687,648 A1. In the known device, the reaction mixture is fed to a first reactor, in which only partial conversion of the methanol is desired. After it has flowed through the first reactor, the gas mixture, which still contains some unconverted starting materials, is guided to a second reactor which is constructed optimally for the residual conversion. The reactors are designed as plate or bed reactors in which the catalyst is provided in the form of a bed or a coating of the dispersion passages. Furthermore, catalysts in the form of coated metal sheets, lattices and foams through which the reaction mixture flows are known.
EP 0,217,532 B1 has disclosed a process for the catalytic generation of hydrogen from mixtures of methanol and oxygen using a gas-permeable catalyst system in which a hydrogen generator is provided with an upper reaction zone and a lower reaction zone, the reaction mixture of methanol and oxygen being fed into the upper reaction zone. After it has flowed through the upper reaction zone, the reaction mixture is guided into the lower reaction zone, in which, as a result of spontaneous initiation of the oxidation of the methanol, the temperature rises to such an extent that partial oxidation of the methanol begins in the upper reaction zone in the presence of a copper catalyst and hydrogen is formed.
Working on the basis of this prior art, the invention is based on the object of providing a device of the generic type which has as simple and compact a structure as possible and in which the amount of catalyst material required for the conversion of a specific mass flow of fuel is minimized. A further object of the invention is to specify a process for producing a catalyst which enables the said minimization of catalyst material and the simple and compact structure to be achieved.
To achieve this object, the invention proposes a device for carrying out a heterogeneously catalysed reaction. Consequently, the device according to the invention comprises a catalyst which is formed by compressing catalyst material into a thin, large-area layer, it being possible to press the reaction mixture through the catalyst with a pressure drop. In contrast to the known devices, such as hydrogen reactors and the like, the catalyst is not designed as a simple surface structure, around which the reaction mixture simply flows, but rather as a highly compressed three-dimensional layer through which the reaction mixture is pressed with considerable pressure applied. The result is a high utilization of the capacity of the active catalyst centres and a high reaction rate at the centres. Due to the considerable pressure drop while the reaction mixture passes through the catalyst layer according to the invention, the flow resistances to the supply and removal of the starting materials and products of the reaction do not play any major role, so that the supply and removal of the substances involved in the reaction can be of simple form. The considerable compression of the catalyst material produces a highly compact catalyst layer, with the result that the proportion of the total volume and weight of the reactor which is formed by the gas space and solids which are not catalytically active (such as for example metal support sheets and the like) is considerably reduced compared to known devices. Preferably, the catalytic material used is fine-grained catalyst granules or powder. In this way, good mass and heat transfer to and from the inner areas of the catalyst grains is ensured even at high reaction rates. Moreover, the proportion of pores through which the mixture can flow increases as the grain size decreases, i.e. the number of xe2x80x9cblind alleysxe2x80x9d for the gas flow decreases. Flowing through the layer imposes a high level of turbulence on the gases, with the result that the film diffusion resistances around the grains of the catalyst material are reduced, leading to improved heat transfer through convection.
In one configuration of the invention, the catalyst layer is arranged substantially at right angles to the direction of flow of the reaction mixture. The result is particularly short paths for the gas to flow through. Due to the large-area, highly compressed configuration of the catalyst layer according to the invention, in the event of the gases flowing at right angles, even a short distance is sufficient to achieve a high level of reaction with a high pressure drop.
In a particularly advantageous configuration of the invention, the catalyst material is compressed with a support structure, with the result that the catalyst material is mechanically stabilized and/or the conduction of heat is improved. The support structure is advantageously a three-dimensional lattice-like structure (matrix), which in a further advantageous configuration of the invention is a metallic support structure. The metal used is, for example, copper, in particular dendritic copper.
In an advantageous configuration of the invention, the catalyst material contains a precious metal, in particular platinum. The added precious metal, which is preferably platinum, although the use of other precious metals is also possible, reacts even at relatively low operating temperatures and thus serves to heat the catalyst arrangement. This measure significantly improves the cold-start performance of the catalyst arrangement, which is advantageous in particular for use in the mobile hydrogen generation sector.
In a particularly advantageous refinement of the invention, a plurality of layers which are connected in parallel are provided. This allows the total surface area through which the reaction mixture is to flow to be spread over a plurality of layers which are arranged one behind the other but are connected in parallel. This xe2x80x9cmodular designxe2x80x9d results in a particularly compact structure of the catalyst arrangement.
To simplify the supply and removal of the substances involved in the reaction, in a further configuration of the invention passages for guiding starting materials of the reaction mixture and the reaction products are provided in the at least one catalyst layer.
In a further configuration of the invention, oxygen, which may promote or be required for the reaction, is fed to the reaction mixture only at the level of the at least one catalyst layer.
According to the invention, to produce a catalyst which can be used in particular in a hydrogen generation device according to the invention, a highly compressed layer which forms a shaped body is formed from at least one catalyst powder by compression, the catalyst powder comprising dendritic copper in powder form.
In one configuration of the invention, the shaped body is sintered following the compression, resulting in particularly high strength of the catalyst according to the invention.
In a further configuration of the invention, passages for guiding starting materials and products of the catalytic reaction are formed in the shaped body during compression. Advantageously, these passages are produced by the introduction of spacer elements which can be removed again in a subsequent process step. The spacer elements are advantageously removed by being burnt, pyrolysed, dissolved or vaporized.
In a further advantageous configuration of the invention, a further powder layer is pressed onto a ready-sintered shaped body and is then sintered. This allows a catalyst with a plurality of layers positioned one above the other to be produced in a type of sandwich structure in a multistage production process, which layers are connected in parallel by suitable passages being formed. As a result, the total catalyst volume through which the reaction mixture is to flow can be spread over a smaller cross-sectional area while nevertheless maintaining the concept of the high pressure drop over a short flow path.